Composite materials are of great current interest because they provide a very favorable combination of high strength and low density. Typically, a composite material is comprised of fibers of graphite, boron, glass, and the like embedded within an epoxy, phenolic or other polymer resin matrix. The more advanced composites which have particularly favorable high strength to density ratio properties are especially attractive for aerospace applications. But typical of other advanced aerospace materials they present comparative processing difficulties; they cannot be made by a simple layup of the fibers and resin followed by room temperature curing. Aerospace composite materials not only involve more difficult-to-fabricate resins but often essentially defect-free finished parts must be produced. As a result, aerospace composites are typically molded and cured at elevated temperatures under substantial pressure.
One method of molding (pressure pads) comprises using shaped pads of a high thermal expansion silicone rubber. An uncured prepreg is contained within a space between abutting adjacent pads and the assembly is captured in a closely fitting closed metal vessel. The vessel and contained assembly are then heated to an elevated temperature to both cure the article and expand the rubber to apply pressure to the article during its cure. The pressure pads are also referred to as trapped rubber tooling.
A high degree of thermal conductivity is often desired in trapped rubber tooling. The increased conductivity permits more rapid heating of articles during pressurization. Aluminum powder is generally used because it has good thermal conductivity properties, is low in density, and is low in cost. Metal additives, however, lower the amount of polymer in the compound with consequent lower thermal expansion capability. The metal also increases the hardness of the tooling rubbers reducing its compliance to irregular articles under pressure.
Although a variety of molding processes have been used to mold composites at elevated temperatures and pressures (e.g. compression molding, isostatic pressure molding using pressure bags or pressure vessels, pressure pad molding) there are problems (e.g. bag leaks) associated with these processes.
Accordingly, there is a constant search for composite molding processes that are not subject to pressure loss if minor flaws occur in the barrier between the pressurant and the article.